I would suggest putting together a list of your favourite domains and see where you can have Machine Learning making it more useful.
Share some of your personal interests and let the community do it’s thing…;)
Anyways, one idea could be making an eBooks app that learn user’s behaviour for reading books, novels etc and based on that suggest new more interesting content for that user…similar work is there for movies, videos, blogs and news but I haven’t seen anything done specific to e-books. e-ink tablet markets will kill to get something effective like this. :D